Brake Squeal

Dixcel – Brake Technical ‘Brake Squeal’

This is a technical article regarding Brake Squeal.

After brake pad replacement, squeals frequently occur. While it is a well-known problem, the cause of the squeal is usually unclear. Also, literature regarding the squeal is seldom available to study leading to the dead end stage.

Here, we would like to introduce a troubleshooting guide for you to understand the squeal mechanism, causes, and measures you can take to reduce or eliminate brake squeal.

Brake squeal happens when vibration occurs between the disc and pad. Due to the shape of the disc, it can act as an amplifier, causing the vibration to sound very loud and irritating. Brake squeal can be explained in the illustration below.

When the brake is applied, the pad is pushed by a piston in the caliper; this causes the pad to make contact with the disc. If there is an even distribution of pressure applied across the whole pad, then squeal is less likely to occur. When the pads aren’t placing even pressure on the disc, vibration (or judder) can occur, causing high pitched squeal. Such vibrations will be amplified by the disc and can transfer to the caliper and suspension, and in worst cases, the vehicle body.

Many factors can contribute to the effectiveness of vibration absorption, such as:

Pad material softness (attenuation characteristics)

Disc softness (attenuation characteristics)

Shim between pad backing plate and piston

Should absorption not be effective, vibration will turn in to a loud and unpleasant noise.

It is obvious that brake noise can initially be generated by pad and disc contact, but that may not be the root of the problem. It is important to find the real cause of the problem. The following illustrations are examples of what may be causing your brakes to squeal.

Case Study 1 on brake squeal

Case 1

In this case, the old disc was used with a set of new pads. Immediately after replacement of the pads, the squeal started. It would be easy to assume that the pads were to blame seeing as the squeal only started after the pads were replaced, but on closer inspection, we notice that the disc has a sharp edge from where it has been worn down due to the previous set of pads.

Case 2

This is a similar case where the old disc was used with a new set of pads. As with the previous case, squeal started right after the pads were replaced. First we inspect the pads and notice no abnormality on the surface, then on inspection of the disc we notice that there is friction material and deposits left over from the previous set of pads causing grooves to form in the surface.

Conclusion

Firstly, we noticed that the disc in Case 1 was worn out and had a sharp chamfer formed on the edge of the disc. When the new pad was installed and the brakes applied, the flat surface of the new pad was being scratched by the sharp edge like a nail and hence causing the squeal. Only a very small section of the pad was touching the disc which also made for dangerous braking.

In Case 2, we notice that material from the previous pads had built up on the old disc, causing grooves and an uneven surface. This causes uneven distribution of pressure on the disc and pad, causing vibration and hence squeal.

So, as you can see, it is easy to assume that the new pads may be the cause of the problem as the squeal generates immediately after installation, especially if your brakes didn’t squeal beforehand. This is because the old brake pads were worn in at the same time as the disc, so the pads and disc have the same wear patterns on them.

To continue, we would like to explain to you how the caliper and piston relate to brake squeal. The caliper and piston may also be related to other brake problems you may be having but are often overlooked. Take a look at the diagram below.

As the diagram shows, the piston is held in the cylinder body by the piston seal. The elasticated piston seal is what pulls the piston and pad back into the cylinder when the brake is released. Over time, however, the seal becomes worn and loses its elastic properties, and the pad doesn’t release from the disc, causing drag. Vibration absorption through the pad and disc is greatly reduced and the vibration can easily transfer to the caliper itself, encouraging squeal.

Now we know that the piston seal and dust boot are both important components, and when they deteriorate, various problems can occur. However, most people don’t know when to replace them. Generally, it is advised that you replace the seals once every 100,000km, depending on environment and driving style (commuting, race, off-road etc.). For those who race, more frequent seal replacement is advised. It is best to replace once every five running events for circuit run drivers. For those who’ve never replaced their brake seals and are hearing noise coming from their brakes, we recommend replacing the seals.

You may be surprised to find that such parts can cause brake squeal, but seeing these deteriorated parts may help you understand how fundamental they are to the smooth functioning of your brakes. Also, while you’re replacing worn parts, it won’t hurt to check over any parts that aren’t being replaced to make sure they are in good condition.

When brake squeal occurs ...

Many factors can cause brake squeal to occur. See chart and list below to help pinpoint the cause of brake squeal.

Vehicle model

Pad / Disc part #

Installation period, run period after installation, overall run distance